Radiation sensitive condition responsive system

ABSTRACT

A condition responsive system comprises a condition sensor for producing a fluctuating output signal having a predetermined frequency component in response to sensing of the condition to be detected, an output device arranged to assume a first state indicative of the presence of the condition being sensed when the sensor senses the condition and produces the fluctuating output signal and a second state indicative of the absence of the condition when the sensor does not produce the fluctuating output signal, and means responsive to the fluctuating output signal and connected in circuit between the sensor and the output device including an inductor for blocking signals of higher frequency than the frequency of the fluctuating output signal, a capacitor connected in series with the indicator for blocking signals of lower frequency than the frequency of the fluctuating output signal and a fail safe rectification circuit connected in circuit between the series combination of the capacitor and the inductor and the output device.

United States Patent Thomson Mar. 21, 1972 [54] RADIATION SENSITIVECONDITION Primary ExaminerWalter Stolwein RESPONSIVE SYSTEMAttrneyWillis M. Ertman [72] Inventor: Elihu Craig Thomson, Wellesley,Mass. [57] ABSTRACT ['73] Assignee: E FZ T Corporation of America Acondition responsive system comprises a condition sensor n for reducinga fluctuating output signal having a redeter- P I P [22] Filed: Aug. 25,1970 mined frequency component in response to sensing of the conditionto be detected, an output devicea'rranged to assume a [2]] Appl' 66841first state indicative of the presence of the condition being sensedwhen the sensor senses the condition and produces the [52] 11.8. CI...250/217 F, 250/214 R, 431/79, fluctuating output signal and a secondstate indicative of the 34 /2282 absence of the condition when thesensor does not produce [51] InLCl. ..G08b21/00 the fluctuating outputsignal, and means responsive to the [58] Field of Search ..250/217 F,214, 206; fluctuating output signal and connected in circuit between the340/2282; 431/79; 307/233 231 sensor and the output device including aninductor for blocking signals of higher frequency than the frequency ofthe [56] References Cned fluctuating output signal, a capacitorconnected in series with UNITED STATES PATENTS the indicator forblocking signals of lower frequency than the frequency of thefluctuating output signal and a fail safe 2,307,008 1957 ROWell Xrectification circuit connected in circuit between the series I 9 1,5401 1/1959 Powers --250/217 F combination of the capacitor and theinductor and the output 3,154,724 10/1964 Gioffrida... ...340/228.2 Xdevice. 3,568,074 3/1971 Carroll ..307/233 X 9 Claims, 1 Drawing Figure[5K /0K 2/V5366 5 6 2/1/5232 74 l4 :0 F\ 34 39K z/vszaz l lin. //5 v2.2% .m" m HENRY Ir 84 I2 I6 f 022 r 46 g )r I4 52/ /a0/( {j 221% 76 72A r 44 M232 88 7 2/V5232 2/v5252 Z 32 Alf RADIATION SENSITIVE CONDITIONRESPONSIVE SYSTEM SUMMARY OF INVENTION This invention relates toimproved control apparatus and more particularly to condition responsivesystems such as those useful in supervising fuel burning systems, and tomeans for insuring fail safe operation of such systems should acomponent thereof fail.

In condition responsive systems of the type employed for the supervisionof flame in a combustion chamber, the system arrangement must reliablyand accurately indicate the presence or absence of flame, the absence offlame being promptly detected so that the fuel valve may be closedquickly thus preventing an excessive amount of unburned fuel fromaccumulating in the combustion chamber. Malfunction of a component ofsuch a system may cause the flame detecting system to falsely indicatethe presence of flame and thus create an unsafe condition in the eventof flame failure as the system would continue to react as if flame werepresent.

It is an object of this invention to provide a novel and improvedcondition responsive system in which failure of a component will notcause the system to be placed in an unsafe condition.

Another object of the invention is to provide a novel and improvedcombustion supervision system particularly useful with flame sensorsresponsive to infrared radiation.

In accordance with the invention there is provided a conditionresponsive system comprising a condition sensor for producing afluctuating output signal having a predetermined frequency component inresponse to sensing of the condition to be detected, an output devicearranged to assume a first state indicative of the presence of thecondition being sensed when the sensor senses the condition and producesthe fluctuating output signal and a second state indicative of theabsence of the condition when the sensor does not produce thefluctuating output signal and means responsive to the fluctuating outputsignal and connected in circuit between the sensor and the output deviceincluding an inductor for blocking signals of higher frequency than thefrequency of the fluctuating output signal, a capacitor connected inseries with said inductor for blocking signals of lower frequency thanthe frequency of the fluctuating output signal and a fail saferectification circuit connected in circuit between the seriescombination of the capacitor and the inductor and the output device.

In a preferred embodiment the system includes frequency selective meanswhich produces a control signal only when signals applied theretoinclude the predetermined frequency component, and coupling means forapplying output signals produced by the condition sensor to thefrequency selective means. The frequency selective means produces afluctuating control signal which alternately applies positive andnegative voltages to the inductor. The resulting alternation is passedas an AC signal by the capacitor and is rectified and energizes thecontrol device which in the preferred embodiment is a relay.

The sensed condition is fluctuating, either by nature or by electricalmodulation and the system is made fail safe by passing the resultingfluctuating signal through the series inductor capacitor circuit andthereafter rectifying it. In the preferred embodiment, an amplifier isforced to preserve the fluctuating character of the signal while itamplifies it sufficiently to actuate the output device. Neither a shortnor an open circuit can simulate the condition signal for either onewill render the amplifier unable to pass amplify the fluctuating signalso that either type of failure is a safe failure, that is the outputdevice will indicate the absence of the condition to be sensed.Furthermore, in this dynamic system, a static leakage condition incomponents cannot of itself generate a dynamic signal that can simulatethe condition to be sensed. Should the capacitor in the series circuititself develop a short circuit or leakage and pass current directly fromthe supply to the fail safe rectification circuit and possibly to theoutput device, the rectification circuit will either shunt or block thatcurrent flow. If the rectification circuit fails, by either open orshort circuit or leakage, rectification will be entirely orsubstantially reduced so that the output device will either be partiallyor completely deactuated.

In particular embodiments it is preferred to provide a first shunt diodeand a second diode in series between the capacitor and the output deviceand a second capacitor in shunt across the output device so that thecharge required by the first capacitor is transferred to the secondcapacitor while the shunt diode is blocking and the second capacitor canthen be large enough to produce a steady relay current and in addition adeactuation delay.

The series inductor in the series circuit effectively blocks powerfrequency signals (e.g., 60 Hertz) and signals of frequency below thepass band of the series circuit will be blocked by the capacitor. Ifthere is sufficient signal from the condition sensor which in aparticular embodiment is a lead sulfide flame sensor, the relay will beheld in from the signal within the pass band despite the presence ofinterfering signal. If there is insufficient signal, however, thepresence of an overriding spurious signal will block the response to thefluctuating condition signal and the relay will drop out in a fail safemanner.

Other objects, features and advantages of the invention will be seen asthe following description of a particular embodiment progresses, inconjunction with the drawing, in which:

The single FIGURE is a schematic diagram of a system for supervising theestablishment and maintenance of flame in a supervised combustionchamber.

DESCRIPTION OF PARTICULAR EMBODIMENT The circuit shown in the drawingincludes a transformer 10 having a primary winding 12 which a 115 volt,60 Hertz signal is applied at terminals 14. Its secondary winding 16 isconnected to a full wave rectifier 18 which applies the rectified signalbetween positive bus 20 and negative bus 22. Capacitor 24 and resistor26 are connected between buses 20 and 22. Connected to bus 20 byresistor 28 is a subsidiary bus 30 on which the voltage is maintained atplus 7 volts by Zener diode 32. Connected between buses 30 and 22 is avoltage divider network including resistor 34 and a lead sulfidephotoresistive flame sensing cell 36 which is arranged to sense theexistence of flame in the supervised combustion chamber. Such flame hasa fluctuating characteristic within a 5 to 25 Hertz range whichcharacteristic is sensed by photocell 36.

The fluctuating output signal from photocell 36 is applied to a firstamplification stage that includes transistor 40 and a bridge T nullfilter in the feedback connection from the collector 42 of transistor 40to its base electrode 44. The filter network includes capacitors 46 and48 and resistors 50 and 52. The output of the first amplifier stage isdirect coupled to a second amplification stage that includes transistors54 and 56 and another bridge T filter that provides degenerativefeedback to the emitter 58 of transistor 54. That second filter includescapacitors 60 and 62 and resistors 64 and 66. Each filter has a passband within the 5 to 25 Hertz range of the output signal from photocell36.

The amplified fluctuating flame signal is then coupled by capacitor 70to a switching circuit that includes transistors 72, 74 and 76. In thequiescent condition, transistor 72 draws base current through resistor78 from the positive supply bus 20 so that transistor 74 is cut off andtransistor 76 is saturated. When the collector of transistor 56 goespositive it charges capacitor 70, drawing base current in transistor 72.On the negative swing of the collector of transistor 56, capacitor 70couples that signal and transistors 72 and 76 are cut off and transistor74 draws base current through resistor 80 and is switched intoconductive state. The result is that the input end of inductor 82 isalternately switched between the positive bus 20 and the negative bus 22in response to the fluctuating flame signal.

lnductor 82 and capacitor 84 form a series circuit tuned to thefrequency of the fluctuating flame signal. Inductor 80 is made so thatthere is a fixed spacing between input and output leads and fixedturn-to-turn and layer-to-layer spacing, and therefore the inductor isas safe from short circuit failure as any two leads in the circuitwiring. lnductor 80 also has enough resistance so that it will limit thecurrent to the shunt diode 86 in case of a short circuit in capacitor84. Diode 86 is connected in shunt and diode 88 is connected in seriesbetween capacitor 84 and coil 90 of output relay. Capacitor 92 isconnected across the coil 90. As the input end of inductor 82 isalternately switched between the positive and nega: tive buses 20 and 22at the frequency of the sensed flame condition, typically about Hertz,the resulting alternation is passed as an AC signal by capacitor 84 andis rectified and charges capacitor 92 so that relay 90'is pulled in.

If a very large 60 Hertz signal is imposed on the detector input lead,the filters will be insufficient to block it and as a result transistors74 and 76 will impose on inductor 82 a 60 Hertz signal whose peak topeak value is represented by the power supply voltage. With thecomponent values shown, the inductor effectively blocks this signal andnegligable current will be rectified and delivered to the relay coil.Similarly, frequencies below the pass band of the series circuit ofinductor 82 and capacitor 84 will be blocked by the capacitor 84.Capacitor 92 provides a 1% second drop out time delay so that the flameflicker signal must be absent for a number of cycles before the relaywill drop out.

It will be seen that this circuitry provides discrimination againstsingle component failure and AC signals other than the desired conditionsignal in a simple, economical and reliable circuit arrangement. While aparticular embodiment of the invention has been shown and described,various modifications will be apparent to those skilled in the art andtherefore it is not intended that the invention be limited to thedisclosed embodiment or to details thereof and departures may be madetherefrom within the spirit and scope of the invention as defined in theclaims.

What is claimed is:

1. A condition responsive system comprising a radiation sensitivecondition sensor for producing a fluctuating output signal having apredetermined frequency component in response to sensing of thecondition to be detected,

an output device arranged to assume a first state indicative of thepresence of the condition being sensed when said sensor senses saidcondition and produces said fluctuating output signal and a second stateindicative of the absence of the condition when said sensor does notproduce said fluctuating output signal, and

means responsive to said fluctuating output signal and connected incircuit between said sensor and said output device including an inductorfor blocking signals of higher frequency than the frequency of saidfluctuating output signal, a capacitor connected in series with saidinductor for blocking signals of lower frequency than the frequency ofsaid fluctuating output signal and a rectification circuit connected infail safe cooperation between the series combination of said capacitorand said inductor and said output device.

2. The system as claimed in claim 1 wherein said rectification circuitincludes a rectifier device connected in shunt with said capacitor.

3. The system as claimed in claim 2 wherein said rectification circuitfurther includes a second rectifier connected in series with saidcapacitor and a second capacitor connected in shunt across said outputdevice.

4. The system as claimed in claim 1 and-further including frequencyselective means for producing a control signal only when signals appliedthereto include said predetermined frequency component, said frequencyselective means including a band pass amplifier tuned to pass signals inthe 5 to 25 Hertz range. and coupling means for applying output signalsproduced by said condition sensing means to said frequency selectivemeans.

5. The system as claimed in claim 1 wherein said condition to bedetected is flame in a combustion chamber and-said output device is thecoil of a relay.

6. The system as claimed in claim 5 wherein said sensor is a leadsulfide photocell.

7. The system as claimed in claim 6 wherein said rectification circuitincludes a rectifier device connected in shunt with said capacitor.

8. The system as claimed in claim 7 and wherein said rectificationcircuit further includes a second rectifier connected in series withsaid capacitor and a second capacitor connected in shunt across saidoutput device.

9. The system as claimed in claim 8 and further including frequencyselective means for producing a control signal only when signals appliedthereto include said predetermined frequency component, said frequencyselective means including a band pass amplifier tuned to pass signals inthe 5 to 25 Hertz range, and coupling means for applying output signalsproduced by said condition sensing means to said frequency selectivemeans.

IOIOZB 0689

1. A condition responsive system comprising a radiation sensitivecondition sensor for producing a fluctuating output signal having apredetermined frequency component in response to sensing of thecondition to be detected, an output device arranged to assume a firststate indicative of the presence of the condition being sensed when saidsensor senses said condition and produces said fluctuating output signaland a second state indicative of the absence of the condition when saidsensor does not produce said fluctuating output signal, and meansresponsive to said fluctuating output signal and connected in circuitbetween said sensor and said output device including an inductor forblocking signals of higher frequency than the frequency of saidfluctuating output signal, a capacitor connected in series with saidinductor for blocking signals of lower frequency than the frequency ofsaid fluctuating output signal and a rectification circuit connected infail safe cooperation between the series combination of said capacitorand said inductor and said output device.
 2. The system as claimed inclaim 1 wherein said rectification circuit includes a rectifier deviceconnected in shunt with said capacitor.
 3. The system as claimed inclaim 2 wherein said rectification circuit further includes a secondrectifier connected in series with said capacitor and a second capacitorconnected in shunt across said output device.
 4. The system as claimedin claim 1 and further including frequency selective means for producinga control signal only when signals applied thereto include saidpredetermined frequency component, said frequency selective meansincluding a band pass amplifier tuned to pass signals in the 5 to 25Hertz range, and coupling means for applying output signals produced bysaid condition sensing means to said frequency selective means.
 5. Thesystem as claimed in claim 1 wherein said condition to be detected isflame in a combustion chamber and said output device is the coil of arelay.
 6. The system as claimed in claim 5 wherein said sensor is a leadsulfide photocell.
 7. The system as claimed in claim 6 wherein saidrectification circuit includes a rectifier device connected in shuntwith said capacitor.
 8. The system as claimed in claim 7 and whereinsaid rectification circuit further includes a second rectifier connectedin series with said capacitor and a second capacitor connected in shuntacross said output device.
 9. The system as claimed in claim 8 andfurther including frequency selective means for producing a controlsignal only when signals applied thereto include said predeterminedfrequency component, said frequency selective means including a bandpass amplifier tuned to pass signals in the 5 to 25 Hertz range, andcoupling means for applying output signals produced by said conditionsensing means to said frequency selective means.